Seed Sowing and Soil Temperature

The forecast shows we are about to have some sunny warm days ahead here in the PNW- thank goodness.  But I want to warn you not to go out and so your whole Spring Garden this weekend!  The temperatures will rise but the soil takes quite a bit longer to warm up so please be patient.

Getting the timing right to sow seeds and set out transplants is key to a successful annual vegetable and flower garden.  Following the calendar helps of course, but also more specifically, tracking soil temperatures can help you be more successful in your efforts.  

Sowing seeds “too early” really just means that the soil isn't warm enough to support healthy germination.  Setting out transplants "too early" really means that the soil temperature isn't yet high enough to establish roots and support healthy microbial action.

As the climate becomes more unstable and temperature patterns are more unpredictable, I have started using soil temperature rather than the calendar as my indicator for sowing and transplanting.

To measure soil temperature, I use a soil thermometer but a compost thermometer works well too.  You can find these at your local garden center.  Place the tip of the thermometer about three inches below the surface of the soil to get your reading because that’s where your seeds will be.  Leave it in for a few minutes- go have a wander and come back to it. I measure soil temps weekly and write them down on the calendar so I can compare. Today, my soil reads 53 degrees at noon. These readings also turn into a record of your specific growing area and micro climates within your garden. Another way to know your place.

How do you apply this information to your sowing and transplanting schedule? I found this resource put together by Fruition Seeds to be incredibly helpful. They created these Vegetable, Herb, and Flower Germination Temperature Charts to show various crops and their idea soil temperature range for optimal germination. The charts also include the average days to germination which can be helpful as well. I have been following the soil temp recommendations listed here for a several years and find them spot on. Leave them a donation like I did if you appreciate their work!

When you align your sowing and transplanting calendar with your soil temperature readings I hope you find more success with your seeds.

Let me know if you do this or plan to try it! I'd love to know how it works for you.